The present invention relates to an electrophotographic digital copier for sequentially writing bicolor image data signals in a photoconductive element and producing a bicolor hard copy by a single image transfer.
An electrophotographic image forming apparatus usually includes a photoconductive element in the form of a drum. While the drum is in rotation, a charger uniformly charges the surface of the drum to predetermined polarity. Image data is optically written on the charged surface of the drum to electrostatically form a latent image thereon. A developing device is driven substantially in synchronism with the rotation of the drum to develop the latent image formed on the drum by a toner. Thereafter, an image transferring device transfers the developed image or toner image to a recording medium implemented as a paper sheet. The problem with this kind of image forming apparatus is that when the ratio of an image represented by image data is small, the developing device simply idles over a substantial period of time without developing the image since it is driven substantially in synchronism with the drum. The idling results in various undesirable occurrences such as the deterioration and scattering of the developer, accidental deposition of the toner on reproduced images, and the decrease in the life of the apparatus due to degraded functions. To eliminate this problem, Japanese Patent Laid-Open Publication No. 134660/1990 teaches an image forming apparatus having a developing device which is spaced apart from an image writing position by a distance associated with a period of time necessary for the developing device to fully rise. Specifically, in the apparatus disclosed in this Laid-Open Publication, a clutch for driving the developing device is coupled when image data appears for the first time in each page, thereby causing a motor to drive the developing device. At the end of the page of interest or on the elapse of a predetermined period of time, the clutch associated with the developing device is uncoupled. The motor is shared by the developing device and photoconductive drum, i.e., it is also connected to the drum by another clutch.
The apparatus having the above construction, however, has some problems left unsolved, as follows. Locating the developing device at the above-mentioned distance from the image data writing position is undesirable when it comes to miniature apparatuses. Since the clutch associated with the developing device is coupled when image data appears for the first time in each page and uncoupled at the end of the page or on the elapse of a predetermined period of time, the developing device is not free from substantial idling when an image exists only at the leading edge portion of a page. Especially, assume such an anti-idling scheme is applied to a developing device of a copier of the type producing a bicolor hard copy, e.g., a red developing device of a copier capable of producing a bicolor hard copy in black and red. Then, the conventional anti-idling scheme cannot achieve the expected advantage when a date stamp or a seal impression is present at the leading edge of an image. Further, since the developing unit and drum share a single motor, turning on and off the developing device via the clutch causes the load acting on the motor to sharply change with the result that the rotation of the drum is effected to disturb the image.
There has been proposed a digital copier having a reading device for receiving an image light from a material to be copied to produce black image dater and separating image data of particular color, e.g., red image data from the black image data, a first writing device for writing the black image data in a photoconductive element to electrostatically form a latent image, a first developing device for developing the latent image to produce a toner image, a second writing means for writing, after the development by the first developing device, the red image data in the photoconductive element after delaying it a predetermined period of time to thereby form a latent image over the toner image, a second developing device for developing the latent image formed by the second writing device to produce a toner image, an image transferring device for transferring the two toner images to a paper sheet at the same time, and a driving device for driving the developing devices from the beginning to the end of a copying operation. The continuous drive of the developing devices from the beginning to the end of a copying operation is effected so that red image may be written and developed whenever it appears. This also gives rise to the idling problem and, therefore, the previously stated undesirable occurrences when the ratio of an image represented by red image data is small. This is especially true with ordinary documents for office use since most of such documents carry information in black thereon. Exceptional documents used in offices and on which information is printed in red are those having red date stamps or red seal impressions, those having red underlines, those having red corrections, etc. A majority of such red portions center around a particular position of a document or are far smaller in area than black portions.